1. Field of the Invention
The present invention relates to a power supply unit for a microwave oven and a high-voltage fuse used therein.
2. Description of the Prior Art
In general, a microwave oven is used to heat food in a heating chamber by sending microwaves generated in a magnetron controlled by a micro-computer. FIG. 1 shows a conventional power supply unit for a microwave oven.
Referring to FIG. 1, the conventional power supply unit includes a high-voltage transducer 10 which receives an AC voltage from a first coil T1 and generates a high voltage of 2000 V in a second coil T2, a high-voltage fuse 20 which is connected to an output port of the high-voltage transducer 10 and prevents an excessively large current, a high-voltage condenser HC which is connected to a port of the high-voltage fuse 20 and charges and discharges a high-voltage current, a grounded high-voltage diode HD which is connected in parallel to the high-voltage condenser HC and absorbs a surge voltage, and a magnetron M which is connected to an output port of the high-voltage condenser HC and radiates microwaves.
In the power supply unit for a microwave oven, in case a short is generated in one of the high-voltage transducer 10, the high-voltage diode HD, the high-voltage condenser HC, and the magnetron M, an excessively large current is generated in the second coil T2 of the high-voltage transducer 10. Then, a fusible member of the high-voltage fuse 20 which is connected to the second coil T2 of the high-voltage transducer 10 and the high-voltage condenser HC is fused by the current, thereby stopping the current flow and preventing the high-voltage transducer 10 from being damaged.
Considering that the operational voltage of the magnetron M is 4 KV, a high-voltage fuse of 5 KV is used in the power supply unit for a microwave oven, and FIG. 2 shows the high-voltage fuse 20 used in the power supply unit.
Referring to FIG. 2, the high-voltage fuse 20 used in the power supply unit includes a pair of cylindrical insulating members 22a and 22b which are engaged with each other, a pair of fuse holders 23a and 23b which are disposed in the insulating members 22a and 22b respectively, a fuse body 20' located between the fuse holders 23a and 23b, and a pair of lead wires 27a and 27b which are electrically connected to ends of the fuse holders 23a and 23b respectively, and a pair of fuse connecting elements 26a and 26b which are press-welded to ends of the lead wires 27a and 27b.
The fuse body 20' includes a conductive fusible member 24 which is inserted into a vacuumed glass tube 21 and a resilient member 25 which is connected to one end of the fusible member. In case the fusible member 24 is broken, the resilient member 25 retracts so that the distance between the broken surfaces of the fusible member 24 are separated by more than a predetermined distance, e.g., 15.5 mm when the operational voltage of the magnetron M is 4 KV.
The cylindrical insulating members 22a and 22b of the high-voltage fuse 20 have lengths sufficient to prevent transmission of the operational voltage of the magnetron M between the conductive portion of the fuse body 20' and the body of the microwave oven. In case the operational voltage of the magnetron M is 4 KV, the cylindrical insulating members 22a and 22b have lengths which can cover the lead wires 27a and 27b by at least 15.5 mm from uncoated portions of the lead wires 27a and 27b.
However, according to the above-mentioned power supply unit for a microwave oven and the high-voltage fuse, since the cylindrical insulating members 22a and 22b have lengths which can cover the lead wires 27a and 27b by at least 15.5 mm from the uncoated portions of the lead wires 27a and 27b, the total length of the assembled insulating members 22a and 22b is the sum of the length of the fuse body 20' plus the lengths of the uncoated portions of the lead wires plus at least 31 mm, so much assembling space is needed when the microwave is mounted.
Further, according to the above-mentioned power supply unit and high-voltage fuse, as shown in FIG. 1, the high-voltage fuse 20 is mounted between a high-voltage end of the second coil T2 of the high-voltage transducer 10 and the high-voltage condenser HC, so both ends of the high-voltage fuse 20 should remain at least 2 KV respectively. Further, two separate insulating members 22a and 22b are needed in the high-voltage fuse 20, so the fuse is long and the manufacturing processes of the high-voltage fuse 20 is complicated.